Switching operations for radio frequency applications can be accomplished by switching devices having a variety of configurations. One of the most common types of switching devices is the single pole single throw (SPST) switch. The SPST switching devices can be combined to perform complex switching operations, and should be able to switch large amounts of power.
One type of switching device commonly used for switching applications is illustrated generally at 6 in FIG. 1. The switching device 6 includes a PIN diode 8 and DC blocking capacitors 10 and 12. Switching device 6 includes inductors 14 and 16 to provide reactive isolation. Inductor 14 is coupled between a bias input 20 and an input 18 of PIN diode 8. Inductor 16 is coupled between a bias input 24 and an output 20 of PIN diode 8. The bias inputs 20 and 24 cause PIN diode 8 to switch from a non-conductive to a conductive state when the voltage difference between bias inputs 20 and 24 is sufficient to forward bias PIN diode 8. When PIN diode 8 is in the conductive state, switch circuit 6 passes an input signal received at an input 26 to output 28.
A disadvantage of this approach is the necessity of providing a constant DC current to forward bias PIN diode 8. The constant current requirements of PIN diode switches can be 10 milliamps or more. This high current requirement can be a particular disadvantage for portable devices which have limited power source availability.
Another type of switching device commonly used for switching applications is illustrated generally at 30 in FIG. 2. Switching device 30 includes a field effect transistor (FET) 32, DC blocking capacitors 34 and 36, and resistors 38 and 40. Bias inputs to FET 32 are provided at bias inputs 42 and 44. Bias inputs 42 and 44 cause FET 32 to switch from a non-conductive to a conductive state when the voltage difference between bias inputs 42 and 44 exceeds the gate to source threshold voltage for FET 32. Switch circuit 30 passes a signal from an input 50 to an output 52 when FET 32 is biased in the conductive state.
A disadvantage of this approach is that the linearity of FET 32 is poor when FET 32 is in either the non-conductive or the conductive state. The poor linearity results from the sensitivity of FET 32 to changes in the drain-to-source voltage observed between lines 46 and 48. When bias input 44 is set to a defined voltage level and FET 32 is in the conductive state, changes in the input signal at 50 can modulate the channel resistance of FET 32 resulting in signal distortion and poor linearity. Distortion can also occur if FET 32 is biased in the non-conductive state and the input signal at 50 causes a drain-to-source voltage which is large enough to put FET 32 back into the conductive state.
In view of the above, there is a need for an improved switch which minimizes signal distortion while requiring minimal current to operate.